A number of unique refrigeration cycles and apparatus have been developed to satisfy the increasing demand for highly reliable, long-lasting cryogenic refrigerators for use in such diverse fields as electronic communications systems, missile tracking systems, super conducting circuitry, high field strength magnets, and medical and biology laboratories for preparation of tissue samples and freezing of solutions. These refrigeration cycles and apparatus, all based upon the controlled cycling of an expansible fluid with suitable heat exchange to obtain refrigeration, are exemplified by U.S. Pat. Nos. 2,906,101, 2,966,034, 2,966,035, 3,045,436, 3,115,015, 3,115,016, 3,119,237, 3,148,512, 3,188,819, 3,188,820, 3,188,821, 3,218,815, 3,333,433, 3,274,786, 3,321,926, 3,625,015, 3,733,837, 3,884,259, 4,078,389 and 4,118,943, and the prior art cited in the foregoing patents.
The present invention is directed at refrigeration systems which employ a working volume defined by a vessel having a displacer therein with a regenerator coupled between opposite ends of the vessel so that when the displacer is moved toward one end of the vessel, refrigerant fluid therein is driven through the regenerator to the opposite end of the vessel. Such systems may take various forms and employ various cycles, including the well known Gifford-McMahon, Taylor, Solvay and Split Stirling cycles. These refrigeration cycles and apparatus require valves or pistons for controlling the flow and movement of working fluid and the movement of the displacer means. The fluid flow and the displacer movement must be controlled continuously and accurately so that the system can operate according to a predetermined timing sequence as required by the particular refrigeration cycle for which the system is designed. Although a fixed timing sequence is the usual objective, it also is desirable to be able to alter the sequence in certain respects, e.g., the time over which high pressure fluid is introduced to the vessel or the time period during which expansion and cooling are achieved.
Heretofore the valving of cryogenic equipment of the type described has taken various forms, but inevitably the valving or the resulting refrigerator has sufferend from one or more of the following limitations: complexity of construction, relatively high cost of manufacture, difficulty of modification as to timing sequence, relatively short operating life, poor reliability, difficulty of adjustment after assembly, and small range of refrigeration capacities. The problem of complexity in construction has been especially great where there have been attempts to achieve self-regulating valve systems. Additional specific problems that have plagued prior cryogenic equipment have been disintegration of lead shot in the regenerator section due to the "slamming" or "banging" of the displacer on its mechanical stops each time it undergoes direction reversal, excessive size of the valving (or of the refrigerator because of the valving construction and/or location), the criticality or short life of seals between certain moving parts, reduced efficiency due to excessive work input or work absorption (e.g. high friction losses), and inability to operate at the low reciprocating speeds that are preferred for such apparatus. Among the several types of valve systems that have been employed are rotary valves as exemplified by U.S. Pat. Nos. 3,119,237, 3,625,015, fluid actuated valves as shown in U.S. Pat. No. 3,321,926, cam operated valves as disclosed by U.S. Pat. No. 2,966,035, mechanically actuated slide valves as shown in U.S. Pat. No. 3,188,821, and displacer-operated valves as shown in U.S. Pat. No. 3,733,837.
U.S. Pat. No. 3,733,837 discloses refrigerators in which cooling of a gas is achieved by expanding it in an expansion chamber, with gas flow to and from the expansion chamber being controlled by a valve having a slidable member operated by the displacer. The refrigerators are self-regulating in the sense that movement of the slidable valve member is controlled by the displacer and movement of the displacer is caused by a gas pressure differential determined by the position of the valve member. The refrigerators disclosed in U.S. Pat. No. 3,733,837 have a number of limitations. First of all the slide valves result in a relatively large void volume which is always filled with gas. Since the gas in the void volume is not cooled, the device has an efficiency limitation. The void volume can be reduced by reducing the diameter of the upper end of the displacer, but since that reduces the effective area it creates the adverse effect of reducing the pneumatic driving force on the displacer. On the other hand increasing the diameter of the upper end of the displacer, as may be desirable for larger capacity refrigerators, is troublesome since that cannot be done without proportionately increasing the overall size of the slide valve. Secondly the fixed portion of the valve is located outside of the refrigeration cylinder while the movable valve member is located inside of the cylinder. Hence the valve does not lend itself to being preassembled as a discrete unit with precision-fitted parts. Still another limitation is that the reciprocating speed of the displacer cannot be varied easily and quickly.